Hitting the light switch for magnetic manipulation

WEST BENGAL, India – Reversing the magnetic properties of materials
is the underlying reality behind computer hard drives, audiotape and other recording
media. For these purposes, changing the magnetic state of macroscale particles suffices,
but efforts are under way to exploit the spins and magnetic moments of electrons,
which could lead to smaller electromagnetic devices than currently exist.

Using the inorganic materials typically chosen for their ferromagnetic
properties limits the amount of time an electronic spin state lasts, however. Organic
chemicals have garnered interest in the nascent field of spintronics because they
can hold spins longer and therefore are more suitable for use in electromagnetic
solid-state devices.

Altering a magnetic field commonly is performed with other, more
highly magnetic fields or with radio-frequency devices, but using photons generally
is easier and would seem well-suited for spintronics. Unfortunately, little to no
work has been done on matching photoinduced magnetization switching techniques with
organic chemicals.

Now, Anirban Misra and his colleague Suranjan Shil of the University
of North Bengal in Siliguri, India, have completed a study of the magnetic properties
of several azobenzene molecules.

When exposed to UV radiation,
molecules in the azobenzene family change isomeric shape, which also changes them
from antiferromagnetic to ferromagnetic. Courtesy of Anirban Misra, University of
North Bengal.
Using imino nitroxide, nitronyl nitroxide and verdazyl –
three highly stable forms of azobenzene – the researchers delved into the
molecules’ ferromagnetic properties in their natural trans isomeric forms.
In the trans form, each azobenzene is antiferromagnetic, or not susceptible to magnetic
fields at all. However, after exposure to UV radiation in the 340- to 380-nm range,
the molecules converted into their cis forms, which proved to be ferromagnetic.

Importantly for potential solid-state data manipulation, this
photoisomerization process takes place at the nanosecond scale, enabling ultraquick
on-off digital data processing. In addition, the ferromagnetic cis forms of the
molecules had highly polarized electron spins, whereas the trans forms did not,
indicating a possible role for cis azobenzenes as spin valves in electromagnetic
read heads or in magnetic field detectors.

The investigators also measured the coupling constant (J) of each
organic molecule, determining that the cis form of verdazyl has the highest J, which
they measured to be 79 inverse centimeters. Nitronyl nitroxide had a J of 64 cm—1
and imino nitroxide the lowest, 15 cm—1. According to Misra, the larger the J value,
the more suitable the molecule would be for potential applications. They reported
their findings in the Feb. 4, 2010, issue of the Journal of Physical Chemistry A.

The scientists are now focused on further quantifying spintronic
behavior in organic molecules as well as on evaluating the properties of “magnetically
interesting” organic and inorganic systems.

“This is the first report of magnetization reversal in the
systems of organic origin,” Misra said. “In organic systems, spins can
be preserved for longer timescales than conventional inorganic materials. This may
eventually lead to the application of photoinduced spintronics, photomagnetic switches,
spin valves and so on.”